ORIGINAL: downunder
I've posted this photo before but here it is again
. This is the head from an OS 40VF with a long reach plug on the left (about half a thread proud) and an OS plug on the right. My preference would be for the long reach plug. Enya plugs are the same length as OS but have a reputation for great reliability.
Is there a known reason for this mid length plug be it Enya of OS? I can't say as I ever investigated the Enya glow plug much.